Conventionally, as a method of broadening the dynamic range of an image sensing element, many proposals have been made. For example, Japanese Patent Laid-Open No. 11-313257 describes an arrangement which outputs a signal corresponding to a logarithm of light that enters a photodiode in a pixel unit. Japanese Patent Laid-Open No. 2000-59688 describes a method of broadening the dynamic range by performing photoelectric conversion by both a photodiode and floating diffusion. Japanese Patent Laid-Open No. 2001-177775 describes a method of broadening the dynamic range by transferring a charge generated by a photodiode to a floating diffusion a plurality of number of times.
FIG. 2 is a circuit diagram showing the arrangement of one pixel in an image sensing element, and that of a circuit for reading out a signal from that pixel. In the image sensing element, a pixel array which provides a two-dimensional image is formed by arranging a plurality of pixels in a two-dimensional array.
Each pixel 201 includes a photodiode (to be also abbreviated as PD hereinafter) 202, transfer switch 203, floating diffusion unit (to be also abbreviated as FD hereinafter) 204, reset switch 207, amplifying MOS amplifier 205, and selection switch 206.
The PD 202 serves as a photoelectric converter that photoelectrically converts light coming via an optical system. The anode of the PD 202 is connected to a ground line, and its cathode is connected to the source of the transfer switch 203. The transfer switch 203 is driven in response to a transfer pulse φTX input to its gate terminal, and transfers a charge generated by the PD 202 to the FD 204. The FD 204 serves as a charge-voltage converter which temporarily accumulates the charge, and converts the accumulated charge into a voltage signal.
The amplifying MOS amplifier 205 serves as a source follower, and its gate receives the signal that has undergone the charge-voltage conversion by the FD 204. The drain of the amplifying MOS amplifier 205 is connected to a first power supply line VDD1 that provides a first potential, and its source is connected to the selection switch 206. The selection switch 206 is driven by a vertical selection pulse φSEL, its drain is connected to the amplifying MOS amplifier 205, and its source is connected to a vertical signal line 260. When the vertical selection pulse φSEL goes to active level (high level), the selection switch 206 of a pixel which belongs to the row of interest of a pixel array is enabled, and the source of the amplifying MOS amplifier 205 is connected to the vertical signal line 260.
The drain of the reset switch 207 is connected to a second power supply line VDD2 which provides a second potential (reset potential), and its source is connected to the FD 204. The reset switch 207 is driven by a reset pulse φRES input to its gate, and resets the charge accumulated on the FD 204.
A floating diffusion amplifier is formed by a constant current source 209 which supplies a constant current to the vertical signal line 260 in addition to the FD 204 and amplifying MOS amplifier 205. In each pixel that forms the row selected by the selection switch 206, a charge transferred to the FD 204 is converted into a voltage signal by it, and is output to a corresponding signal read unit 210 via the floating diffusion amplifier.
A switch 211 is used to read out a reset potential of the FD 204 as a reset level signal, and is driven by a reset level read pulse φTN. A reset level accumulation capacitor 212 accumulates a reset level signal (a signal of a potential corresponding to the reset level of the FD 204) immediately before a pixel signal is read out.
A switch 214 is used to read out a voltage signal corresponding to the charge signal generated by the PD 202, and is driven by a signal read pulse φTS. A signal level accumulation capacitor 215 accumulates a pixel signal (a signal of a potential according to a charge transferred from the PD 202 to the FD 204) when the pixel signal is read out.
A differential amplifier 217 outputs, onto an output line 218, a difference between the level of the signal accumulated on the reset level accumulation capacitor 212 and that of the signal accumulated on the signal level accumulation capacitor 215. Switches 213 and 216 are driven by a horizontal signal selection pulse φHi, and respectively transfer the potentials on the capacitors 212 and 215 to the differential amplifier 217. On the output line 218, a value obtained by amplifying the difference between the potential corresponding to the charge transferred from the PD 202 to the FD 204 and that of the FD 204 in a reset state is output as a pixel signal.
To common output lines 217a and 217b connected to the input terminals of the differential amplifier 217, typically, switches 213 and 216 in other columns, which are driven by horizontal signal selection pulses φH1 to φH(i−1) and φH(i+1) to φHn, are also connected (n is the number of columns of a pixel array 101).
FIG. 4 shows a drive pattern of the image sensing element shown in FIG. 2. During a period t401, the pulses φRES and φTX are applied to turn on the reset switch 207 and transfer switch 203. As a result, the potentials of the PD 202 and FD 204 are reset to initial potentials, and upon completion of the reset operation, a new exposure period starts. After that, the pulse φSEL is applied to turn on the selection switch 206, thus selecting a read row. During a period t402, the pulse φTN is applied to turn on the switch 211, thus writing a value corresponding to the reset potential of the FD 204 in the reset level accumulation capacitor 212.
During a period t403, the pulses φTX and φTS are applied. In response to these pulses, the switches 203 and 214 are turned on. As a result, a charge accumulated on the PD 202 is transferred to the FD 204, and a potential corresponding to the charge transferred to the FD 204 is written in the signal level accumulation capacitor 215. When the pulse φH is applied during a period t404, the switches 213 and 216 are turned on, and the difference between the signal accumulated on the signal level accumulation capacitor 215 and that accumulated on the reset level accumulation capacitor 212 is amplified by the amplifier 217 and is output onto the output line 218.
According to the aforementioned image sensing element, since the difference between the signal level and noise level is amplified and output, fixed pattern noise of the image sensing element is eliminated, and noise due to variations of the reset switches of pixels can also be eliminated.
However, in such image sensing element, when a charge stored in the PD exceeds the parasitic capacitance of the PD, it undesirably leaks into a lower potential barrier portion. Such problems will be discussed with reference to FIGS. 7A and 7B. FIGS. 7A and 7B show the positional relationship among the PD, FD, and transfer switch in an upper portion, and also their potential state in a lower portion. Ideally, all charge components generated by the PD are accumulated on the parasitic capacitance of the PD, as shown in FIG. 7A. However, when charge components generated by the PD are large, they pass under the transfer switch having a low potential barrier, and leak into the FD, as shown in FIG. 7B. In the image sensing element having pixels with the aforementioned structure, when the difference between the signal level and reset level is calculated, as described above, the output signal is proportional to the charge generated by the PD in the ideal case shown in FIG. 7A. However, when the charge leaks from the PD into FD, as shown in FIG. 7B, the output signal decreases by charge components which are generated by the PD and leak into the FD.
Japanese Patent Laid-Open No. 2000-287131 describes a method of preventing the output signal from decreasing by replacing the output signal by a saturated signal after the charge leaks from PD into the FD. Also, Japanese Patent Laid-Open No. 2003-87665 describes a method of broadening the dynamic range by adding the signal corresponding to charge components which leak into the FD to the charge generated by the PD by utilizing this phenomenon.
However, with the method described in Japanese Patent Laid-Open No. 2000-287131, all pixel signals read out from pixels in each of which the charge leaks from the PD into FD are handled as identical pixel values. Therefore, the grayscale characteristics on the high luminance side are lost, and the dynamic range consequently narrows down. Also, with the method described in Japanese Patent Laid-Open No. 2003-87665, an image sensing element in which the charge generated by the PD leaks into only the FD can obtain an output proportional to the incident light amount. However, in an image sensing element in which the charge generated by the PD does not always leak into only the PD, a sensitivity difference is generated between the output based on the charge which leaks into the FD and the output when the PD is not saturated (no charge leaks from the PD). For this reason, their sum output has a knee point when the PD is saturated. When the saturated levels of the PDs and the charge leak amounts from the PDs into FDs have differences for respective pixels, the knee point varies among pixels.